Animation nodes list remove bug?
There are 8 polygon indices in the list, their elements indices ranges between 0 and 7.

When you removed the polygon indices at index 4, you actually removed the 5th element not the the 4th (because indices starts from zero). Anyway, now you have 7 polygon indices that their elements indices ranges between 0 and 6, you are trying to remove the element at index 7 (8th element) which doesn't exist. Thats why there is an error, not sure why it is fatal though.

So, if you want to remove the 5th and 7th element, remove the element at index 4, then remove the element at index 6.